This invention relates to optical sampling arrangements and to spectroscopic apparatus including such arrangements.
Where optical measurements, such as for spectroscopic purposes, need to be made on liquid samples, the sample is usually placed in a transmission liquid cell 1 of the kind shown in FIG. 1. The cell 1 consists of two windows 2 and 3, which are optically transparent to the wavelengths of interest, and which are separated by a spacer 4. The cell 1 is placed in the optical path between a source of optical radiation 5 and a radiation detector 6. The sample 7 to be measured is contained in the cavity 8 between the windows 2 and 3. The pathlength of the cell 1 is determined by the thickness of the spacer 4 and this is arranged to be sufficient so that the sample 7 absorbs a measurable amount of the optical radiation at the wavelengths of interest. The pathlength can vary from several millimetres in the UV and visible regions to just microns in the mid-infrared region.
These liquid transmission cells are widely used but suffer from a number of disadvantages. They can be difficult to fill and are prone to trapping air bubbles, which can prevent accurate measurements being made. Shorter pathlengths, as needed for near and mid-infrared radiation, can be particularly difficult to provide when used with viscous samples. The cells can be very difficult to clean, especially with viscous or sticky samples. This often requires the entire cell to be disassembled and significant quantities of solvent may be needed, which are often flammable or hazardous. The cells usually have a large number of parts, which have to be correctly aligned and assembled to ensure they do not leak. It can also be difficult accurately to reproduce the pathlength when the cell has to be taken apart and rebuilt, such as after cleaning. This is a particular problem when making quantative measurements. Furthermore, the relatively large size of the cell and the large number of components can make it difficult to control or stabilize the temperature of the cell.